Correct dependency checking of plugins

[Jacky Alciné]

Wintermute uses a means of package detection (via QPackageKit) and dependency checking (similar to find_package of CMake) to ensure that the plugin's dependencies are installed (not necessarily properly). Wintermute needs to pull this information and use to provide an efficient means of ensuring plugin preload stability. The following needs to be reached:

1) Definition of package and plugins compatability. A sample of the such (and perhaps a working implementation) is defined here: https://github.com/wntr/plugins/blob/master/marytts-wintermute/spec.in#L16

2) Means of obtaining this information. A setup of it (a simple one at that) is listed here: https://github.com/wntr/core/blob/master/src/plugins.cpp#L203 but can be improved on.

3) Means of validating plugin and package existance. A setup of it is listed here: https://github.com/wntr/core/blob/master/src/plugins.cpp#L211 but can be heavily improved on.

For points 2 & 3, it may be needed to add two new methods to handle packages (i.e: loadPackages(), hasPackages(). Also, a semantic change of the methods from loadPackages() and hasPackages() should be loadPackages() [of which returns a list of package dependency strings], obtainPackages() [of which determines if packages are installed in the system with an optional parameter to install if not found], and hasPackages() [of which determines if all of the packages are installed on the system].

The same should be done for plugins, whereas the installation process should see if it's possible to download them (but that's not a priority).

[Jacky Alciné (jackyalcine)]

So far framework plug-ins can find back-end plug-ins and visa versa. I just have to add QPackageKit support and we'd be fine.